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| United States Patent |
5,191,037
|
|
Doi
, et al.
|
March 2, 1993
|
Biodegradable polymeric composition
Abstract
A biodegradable polymeric composition comprising 100 parts by weight of a
first polymer which is comprised predominantly of R(-)-3-hydroxybutyric
acid and from 3 to 4,000 parts by weight of second polymer which is a
random copolymer of R(--)-3-hydroxybutyric acid and S(+)-3-hydroxybutyric
acid. The composition provides a flexible article having improved
mechanical properties while maintaining biodegradability to a degree at
least comparable to that of the first polymer.
| Inventors:
|
Doi; Yoshiharu (2617-39 Imajuku-cho, Asahi-ku, Yokohama-shi, Kanagawa-ken, JP);
Kumagai; Yoshiharu (Yokohama, JP)
|
| Assignee:
|
Doi, Yoshiharu (Yokohama, JP);
Sumitomo Metal Industries, Ltd. (Osaka, JP)
|
| Appl. No.:
|
778276 |
| Filed:
|
October 17, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
525/450; 525/64; 528/353; 528/361 |
| Intern'l Class: |
C08L 067/04; C08G 063/06 |
| Field of Search: |
525/450,64,190,186,412,415
528/361
|
References Cited
U.S. Patent Documents
| 4393167 | Jul., 1983 | Holmes et al. | 528/361.
|
| 4427614 | Jan., 1984 | Barham et al. | 528/361.
|
| 4477655 | Oct., 1984 | Holmes | 528/361.
|
| Foreign Patent Documents |
| 61-69431 | Apr., 1986 | JP.
| |
Primary Examiner: Kight, III; John
Assistant Examiner: Dodson; Shelley A.
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis
Claims
What is claimed is:
1. A biodegradable polymeric composition comprising 100 parts by weight of
a first polymer which is a microbiologically-produced homopolymer or
copolymer of R(-)-3-hydroxybutyric acid, and from 3 to 4,000 parts by
weight of a second polymer which is a chemically-synthesized random
copolymer of R(-)-3-hydroxybutyric acid and S(+)-3-hydroxybutyric acid.
2. The biodegradable polymeric composition of claim 1, wherein the second
polymer is present in an amount of from 5 to 2,000 parts by weight.
3. The biodegradable polymeric composition of claim 2, wherein the second
polymer is present in an amount of from 10 to 1,000 parts by weight.
4. The biodegradable polymeric composition of claim 1, wherein the first
polymer is a microbiologically-produced homopolymer of
R(-)-3-hydroxybutyric acid.
5. The biodegradable polymeric composition of claim 1, wherein the first
polymer is a microbiologically-produced copolymer of at least 50 mole % of
R(-)-3-hydroxybutyric acid and less than 50 mole % of one or more
copolymerizable monomers.
6. The biodegradable polymeric composition of claim 1, wherein the second
random copolymer contains the recurring units derived from
R(-)-3-hydroxybutyric acid and those from S(+)-3-hydroxybutyric acid at a
molar ratio of the R(-) isomer to the S(+) isomer in the range of from
100:60 to 100:165.
7. The biodegradable polymeric composition of claim 6, wherein the molar
ratio of the R(-) isomer to the S(+) isomer is in the range of from 100:90
to 100:110.
8. The biodegradable polymeric composition of claim 1, wherein the second
random copolymer further comprises one or more additional monomers in a
total amount of at most 10% by weight.
9. The biodegradable polymeric composition of claim 1, wherein the total
weight of the first and second polymers is at least 80% of the total
weight of the composition.
10. The biodegradable polymeric composition of claim 9, which further
comprises one or more additives selected from reinforcing fillers and
various stabilizers including antioxidants, heat stabilizers, and UV
absorbers in a total amount of up to 20% by weight of the composition.
11. A biodegradable article of the polymeric composition of claim 1.
12. The biodegradable article of claim 11, wherein the article is in the
form of a film, sheet, tape, or fiber.
13. The biodegradable article of claim 12, wherein the article is subjected
to drawing so as to improve the mechanical properties.
Description
BACKGROUND OF THE INVENTION
This invention relates to a biodegradable polymeric composition and more
particularly to a polymeric composition which comprises a
poly-R(-)(3-hydroxybutyric acid) or its copolymer produced by
microorganisms and which is completely biodegradable and still has
sufficient mechanical strength for practical uses.
A poly-R(-)(3-hydroxybutyric acid) (hereinafter abbreviated as PHB) is a
biodegradable and biocompatible homopolyester and has prospects for
various applications in which biodegradability or biocompatibility is
required.
For instance, environmental pollution by waste plastics is becoming more
serious and hence there is a great interest in biodegradable plastics.
However, the biodegradable plastics proposed so far are either incapable
of easy and inexpensive production or insufficient with respect to their
mechanical, chemical, or physical properties.
PHB, too, is inadequate with respect to its physical properties in that it
is stiff and brittle, and this problem has obstructed PHB from practical
applications.
It has been proposed that the stiff and brittle nature of PHB can be
alleviated by drawing an article of PHB such as a sheet, film, tape, or
fiber after the article has been pretreated by rolling under pressure or
by heating in a particular temperature range followed by cooling and
maintaining the article at the cooled temperature for a short period
[Japanese Patent Application Kokai (Laid-Open) No. 61-69431(1986)].
However, the improvement in the stiff and brittle nature of PHB by drawing
is temporary and the resulting drawn article tends to recrystallize and
become brittle with time. This tendency is particularly prominent at a
relatively high temperature, e.g., in the range of 30.degree.-80.degree.
C.
Another attempt to improve the nature of PHB relies on addition of a
plasticizer or blending with a different polymer which serves as a
plasticizer to form a polymer blend. For example, a polymer blend of PHB
with a polyethylene oxide is described in Polymer, 29, 1731 (1988) and
that with a polyvinyl acetate is described in Polymer, 30, 1475 (1989).
The alleviation of the stiff and brittle nature of PHB by addition of a
plasticizer including a plasticizing polymer which has been proposed in
the prior art is also unsatisfactory since PHB does not have a sufficient
compatibility with the plasticizer, causing exudation of the plasticizer.
As a result, the PHB article becomes brittle again, and if the article is
left in the field, the pollution of water or soil by the exudated
plasticizer may occur.
For example, when a polyethylene oxide, which is a water-soluble polymer,
is used as a plasticizer, the exudated plasticizer will readily be
dissolved in underground water. Therefore, the usage of this polymer is
strictly regulated.
A blend of PHB with a polyvinyl acetate is still stiff and brittle at room
temperature since the glass transition temperature of a polyvinyl acetate
is about 38.degree. C. which is higher than ordinary room temperatures.
Therefore, the desired improvement in the nature of PHB cannot be
satisfactorily attained by the blend.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to improve the stiff and
brittle nature of PHB while substantially maintaining the biodegradability
thereof.
A more specific object of the invention is to provide a completely
biodegradable polymer blend of PHB with a second polymer which can improve
the undesirable nature of PHB.
The present inventors have found that these and other objects can be
accomplished by blending PHB with a chemically-synthesized polymer of
3-hydroxybutyric acid.
In brief, the present invention resides in a biodegradable polymeric
composition comprising 100 parts by weight of a first polymer which is
comprised predominantly of R(-)-3-hydroxybutyric acid and from 3 to 4,000
parts by weight of a second polymer which is a random copolymer of
R(-)-3-hydroxybutyric acid and S(+)-3-hydroxybutyric acid.
DESCRIPTION OF THE INVENTION
The first polymer is comprised predominantly of R(-)-3-hydroxybutyric acid
and it includes a poly-R(-)(3-hydroxybutyric acid) (PHB), which is a
homopolymer of R(-)-3-hydroxybutyric acid, as well as a copolymer of at
least 50 mole % of R(-)-3-hydroxybutyric acid with less than 50 mole % of
other one or more copolymerizable monomers, both the homopolymer and
copolymer being produced by a microbiological procedure using
microorganisms. These homopolymers and copolymers of R(-)-3-hydroxybutyric
acid are hereinafter collectively referred to as a PHB polymer. Thus, the
PHB polymer comprises from 50 to 100 mole % of recurring units of the
formula: --O--CH(CH.sub.3)--CH.sub.2 --CO-- which are derived from
R(-)-3-hydroxybutyric acid.
Examples of the copolymerizable monomer which may be present in a PHB
polymer are derivatives of R(-)-3hydroxybutyric acid in which one or more
hydrogen atoms in this compound are substituted with an alkyl, halogen,
hydroxy, haloalkyl, hydroxyalkyl, or similar group. Thus, the
copolymerizable monomer produces recurring units of the formula:
--O--CR.sup.1 R.sup.2 --(CR.sup.3 R.sup.4).sub.n --CO-- wherein R.sup.1,
R.sup.2, R.sup.3, and R.sup.4 are independently selected from hydrogen,
halogen, and alkyl, hydroxy, haloalkyl, hydroxyalkyl, and similar groups.
Copolymers of PHB with the above-described one or more comonomers can also
be directly produced by a microbiological procedure using microorganisms.
For instance, an aerobic culture of a mutant of Alcaligenes eutrophus
(NCIB 11599) produces a PHB homopolymer on glucose or a copolymer of PHB
with R(-)-3-hydroxyvaleric acid on a mixture of glucose and propionic
acid, as described in Japanese Patent Application Kokai (Laid-Open) No.
61-69431(1986).
The PHB polymer which is used in the present invention may be either a
crude or purified product isolated from a cell-containing culture of
suitable microorganisms capable of producing a PHB polymer by any known
isolation technique. The origin of the microorganisms used and the
isolation and purification methods employed are not critical and are well
known to those skilled in the art. Typical microorganisms capable of
producing a PHB polymer include Alcaligenes eutrophus, Bacillus
megaterium, Alcaligenes latus, and the like. Also, the cells themselves
which contain a PHB polymer produced by the microorganisms may be used as
the first polymer without isolation of the PHB polymer therefrom.
Commercially-available, microbiologically-produced PHB polymers are also
useful in the present invention.
The first PHB polymer is blended with a second polymer which is a random
copolymer comprised of R(-)-3-hydroxybutyric acid and
S(+)-3-hydroxybutyric acid. The random copolymer is hereinafter referred
to as a racemic PHB copolymer. The racemic PHB copolymer is an atactic
polymer in which recurring units derived from the R(-) isomer of
3-hydroxybutyric acid and those from the S(+) isomer thereof are combined
at random in a polymer chain.
The racemic PHB copolymer generally has a number-average molecular weight
in the range of from 500 to 1,000,000 and a glass transition temperature
below 10.degree. C. It can be prepared by chemical synthesis. The method
of synthesis is not critical in the preparation of the racemic PHB
copolymer and any suitable method can be employed.
By way of example, the racemic PHB copolymer can be prepared by a catalytic
ring-opening polymerization of a mixture of R(+)-beta-butyrolactone and
S(-)-beta-butyrolactone. The mixture is hereinafter referred to as a
racemic BL mixture.
It is preferred that the molar ratio of R(+)-beta-butyrolactone to
S(-)-beta-butyrolactone in the racemic BL mixture used as a starting
material be in the range of from 100:60 to 100:165 and more preferably
from 100:90 to 100:110. Most preferably, this ratio is approximately
100:100. The racemic BL mixture may contain one or more additional
monomers in a minor amount, preferably at most 10% by weight in total
based on the total weight of the monomer mixture. Examples of the
additional monomers which may be present in the racemic BL mixture in such
a minor amount are beta-propiolactone, epsilon-caprolactone, glycolide,
lactide (which may be either an optically-active isomer or a racemic
mixture), and similar lactones.
Consequently, the racemic PHB copolymer prepared from a racemic BL mixture
will preferably contain the recurring units corresponding to
R(-)-3-hydroxybutyric acid and those to S(+)-3-hydroxybutyric acid with
the molar ratio of the R(-) to S(+) isomer being in the range of from
100:60 to 100:165 and more preferably from 100:90 to 100:110, and most
preferably approximately 100:100, while the total weight of these
recurring units preferably comprises at least 90% by weight of the racemic
PHB copolymer.
If the molar ratio of the R(-) to S(+) isomer in the racemic PHB copolymer
does not fall within the above broadest range, the racemic PHB copolymer
may have an increased degree of crystallinity and cannot necessarily
improve the stiff and brittle nature of a PHB polymer sufficiently.
Likewise, in cases where the total weight of the recurring units
corresponding to R(-)- and S(+)-3-hydroxybutyric acid is less than 90% by
weight of the racemic PHB copolymer, the nature of a PHB polymer may not
be improved sufficiently.
A catalyst which is useful in the synthesis of a racemic PHB copolymer from
a racemic BL mixture can be prepared by the method described in
Macromolecules, 10, 275 (1977). According to this method, the catalyst is
made by reacting 1 mole of diethylzinc with 0.6 moles of water in an
inactive solvent such as dioxane in an inert gas atmosphere such as a
nitrogen or argon atmosphere followed by removal of the solvent by vacuum
distillation, for example.
The catalyst thus prepared is added to a racemic BL mixture in an amount of
1 to 5 parts by weight for each 100 parts by weight of the racemic BL
mixture and the mixture is kept at around 60.degree. C. for about 5 days
to allow the ring-opening polymerization of the racemic BL mixture to
proceed. The resulting crude product of a racemic PHB copolymer can be
then purified by dissolving in chloroform, pouring the resulting solution
into diethyl ether, removing the diethyl ether layer by decantation, and
drying the residue in vacuum to give a purified product of the racemic PHB
copolymer.
The above-described method of synthesis of a racemic PHB copolymer is
merely illustrative and other methods can be employed to prepare a racemic
PHB copolymer used in the invention.
It has been found that the racemic PHB copolymer which is blended with a
PHB polymer according to the present invention has a considerable
biodegradability by itself and is completely compatible with a PHB polymer
since it contains a substantial amount of recurring units derived from
R(-) isomer of 3-hydroxybutyric acid which is the major constituent
monomer of the biodegradable PHB polymer. Furthermore, in contrast with a
PHB polymer, the racemic PHB copolymer is a flexible or viscid polymer. As
a result, when it is blended with a PHB polymer, the stiff and brittle
nature of the PHB polymer is improved while substantially maintaining the
biodegradability of the PHB polymer and a completely biodegradable article
which has good mechanical properties and which is free from exudation or
separation of the racemic PHB copolymer from the PHB polymer can be
obtained.
The PHB polymer and the racemic PHB copolymer are blended in a proportion
of from 3 to 4,000 parts, preferably from 5 to 2,000 parts, and more
preferably from 10 to 1,000 parts by weight of racemic PHB copolymer for
each 100 parts by weight of PHB polymer. Less than 3 parts of the racemic
PHB copolymer are insufficient to significantly improve the stiff and
brittle nature of the PHB polymer, while more than 4,000 parts of the
racemic PHB copolymer soften the resulting polymeric composition too much,
thereby adversely affecting the mechanical properties of articles prepared
therefrom.
The polymeric composition may further comprise one or more additives in
addition to the PHB polymer and the racemic PHB copolymer. In such cases,
it is preferred that the total weight of the PHB polymer and the racemic
PHB copolymer comprise at least 80% and preferably at least 90% of the
total weight of the composition. Most preferably, the polymeric
composition consists essentially of the PHB polymer and the racemic PHB
copolymer.
Examples of the additives which may be present in the polymeric composition
of the present invention in minor amounts are reinforcing fillers such as
fiberglass and similar fibers, as well as various stabilizers including
antioxidants, heat stabilizers to prevent thermal degradation, and UV
absorbers. Specific examples of useful stabilizers are oxides of alkaline
earth metals, cuprous iodide, substituted benzophenones, piperidine
derivatives, aromatic amines, and phenols such as
4,4'-bis(2,6-di-tert-butylphenol). If present, these additives are
preferably added in a total amount of less than 20% and more preferably
less than 10% based on the total weight of the polymeric composition.
The PHB polymer and the racemic PHB copolymer can be blended by any known
method, such as by milling under heating or solution mixing, to prepare a
polymeric composition of the present invention.
Milling of a PHB polymer and a racemic PHB copolymer can be performed using
a suitable device such as a roll mill, a pressure kneader, or an extruder
at a temperature which varies from 100.degree. to 200.degree. C. depending
on the proportions of the two polymers and the particular milling device
used.
Solution mixing can be accomplished by dissolving a PHB polymer and a
racemic PHB copolymer together in an appropriate solvent capable of
dissolving both these polymers followed by removal of the solvent. Useful
solvents include chloroform, methylene chloride, 1,2-dichloroethane, and
the like.
The resulting blend, i.e., polymeric composition of the invention can be
shaped into film, fiber, tape, sheet, or similar form by a conventional
technique such as casting or compression molding. The resulting shaped
article is sufficiently flexible for practical applications and withstands
a high degree of stretching. Therefore, it can be drawn at a high draw
ratio to improve the mechanical properties of the article.
In the polymeric composition according to the present invention, the two
polymeric constituents, i.e., the PHB polymer and racemic PHB copolymer
have the same chemical formula but their optical activities are different
from each other. Therefore, they have nearly the same chemical properties
and hence are completely compatible with each other. As a result,
exudation of one of the polymers during or after use and environmental
pollution caused thereby can be completely prevented. Furthermore, due to
the fact that the chemically-synthesized racemic PHB copolymer which is
blended with the microbiologically-produced biodegradable PHB polymer is
also biodegradable to a considerable degree, the polymeric composition is
completely biodegradable and has a high biodegrading rate comparable to or
even higher than the PHB polymer itself.
The polymeric composition is suitable for use as packaging materials as
well as materials for medicine, agriculture, forestry, and fishery.
The following examples describe the invention in more detail.
EXAMPLES
Example 1
I. Synthesis of Racemic PHB Copolymer
(a) Preparation of catalyst
To a 50 ml Schlenk flask which had been purged with nitrogen were added
successively 15 ml of 1,4-dioxane which had been thoroughly purified and
dehydrated and 3.5 ml of diethylzinc in a stream of nitrogen.
Subsequently, 0.37 ml of deoxygenated water was added to the flask over 15
minutes under stirring in a stream of nitrogen at 50.degree. C. The
reaction was allowed to proceed at room temperature for 10 hours and the
1,4-dioxane solvent was then removed by vacuum distillation at room
temperature. The residue was thoroughly dried in vacuum to give 3.5 g of
the desired catalyst as a yellow solid.
(b) Preparation of racemic PHB copolymer
To a 50 ml Schlenk flask were added 0.2 g of the yellow solid prepared
above in a stream of nitrogen and then 7 ml of a racemic BL mixture which
was an equimolar mixture of R(+)-beta-butyrolactone and
S(-)-beta-butyrolactone, also in a stream of nitrogen. The mixture was
reacted for 5 days at 60.degree. C. to give a crude product of the desired
racemic PHB copolymer. The crude product was dissolved in 10 ml of
chloroform and the resulting solution was slowly poured into 100 ml of
diethyl ether with stirring. After stirring for 30 minutes, the mixture
was allowed to stand for another hour and the diethyl ether was then
removed by decantation. The residue was dried in vacuum at room
temperature to give 5.3 g of a purified racemic PHB copolymer. The racemic
PHB copolymer product had a number-average molecular weight of 26,000 and
a weight-average molecular weight of 47,000. The differential thermal
analysis of the product showed that it had a glass transition temperature
of -1.degree. C. and did not have a melting point.
II. Blending of PHB Polymer and Racemic PHB Copolymer
Predetermined amounts of a microbiologically-produced PHB polymer which was
a homopolymer of R(-)(3-hydroxybutyric acid) having a number-average
molecular weight of 362,000 and a weight-average molecular weight of
652,000 and the racemic PHB copolymer prepared above were dissolved in 20
ml of chloroform. The resulting solution was cast onto a petri dish and
allowed to stand for at least 24 hours at room temperature to evaporate
the chloroform solvent. The residue was thoroughly dried in vacuum to give
a 50 .mu.m-thick film of a blend of the PHB polymer and the racemic PHB
copolymer.
The resulting film was subjected to a tensile test in accordance with the
testing procedure specified in JIS K7127 using a #2 test piece at a stress
rate of 2 cm/min to determine the mechanical properties thereof. The test
results are summarized in Table 1 along with the blending ratio.
TABLE 1
______________________________________
Mechanical properties of PHB*/racemic PHB**
Blending Tensile
Ratio Strength Elongation
Sample (wt/wt) (MPa) (%)
______________________________________
PHB -- 38 5
PHB/racemic PHB
95/5 30 100
PHB/racemic PHB
75/25 20 230
PHB/racemic PHB
50/50 12 390
PHB/racemic PHB
25/75 3 >500
PHB/racemic PHB
5/95 2 >500
______________________________________
*PHB: PHB polymer;
**racemic PHB: racemic PHB copolymer.
As shown in Table 1, the stiff and brittle nature of the PHB polymer could
be significantly improved by blending it with a racemic PHB copolymer and
flexible polymeric compositions having an elongation of from 100% to 500%
or higher were obtained.
III. Biodegradability test
The biodegradability of some of the 50 .mu.m-thick films prepared above was
tested in the following manner using 1 cm-square test pieces each weighing
6 mg.
A 0.2 g amount of soil collected in Kashima, Ibaraki-prefecture, Japan was
extracted with 5 ml of sterilized water and a 0.2 ml aliquot of the
extract was added to 5 ml of a culture medium having the composition shown
in Table 2 in weight percent. The resulting dispersion was used as a
testing liquid to evaluate the biodegradability of the test pieces.
TABLE 2
______________________________________
Composition of culture medium (wt %)
______________________________________
0.1% NH.sub.4 NO.sub.3
0.1% KH.sub.2 PO.sub.4
0.1% Na.sub.2 HPO.sub.4
0.02% MgSO.sub.4.7H.sub.2 O
0.01% yeast extract
0.001%
FeSO.sub.4.7H.sub.2 O
0.001% MnSO.sub.4.7H.sub.2 O
0.001% ZnSO.sub.4.7H.sub.2 O
0.001%
CaCl.sub.2.2H.sub.2 O
pH = 7.0
______________________________________
A test piece of each film to be tested was placed into the testing liquid
and shaken for 48 hours at 30.degree. C. Thereafter, the film was removed
from the testing liquid, dried, and weighed to determine the weight loss
as an indication of biodegradability. The test results are summarized in
Table 3 below.
TABLE 3
______________________________________
Biodegradability of PHB*/racemic PHB**
Blending Ratio
Weight Loss
Sample (wt/wt) (mg)
______________________________________
PHB -- 2.1
PHB/racemic PHB
75/25 2.4
PHB/racemic PHB
50/50 2.9
PHB/racemic PHB
25/75 3.5
______________________________________
*PHB: PHB polymer;
**racemic PHB: racemic PHB copolymer.
As is apparent from the results shown in Table 3, all the PHB
polymer/racemic PHB copolymer blends of the present invention had higher
biodegradability than the PHB polymer used as a control.
COMPARATIVE EXAMPLE 1
Following the procedure described in Example 1, the PHB polymer used in
Example 1 was blended with different polymers by solution blending using
chloroform as a solvent to form 50 .mu.m-thick films which were then
tested for mechanical properties. The test results are summarized in Table
4.
TABLE 4
______________________________________
Mechanical properties of comparative PHB blend
Blending Ratio
Tensile Strength
Elongation
Sample (wt/wt) (MPa) (%)
______________________________________
PHB.sup.1)
-- 38 5
PHB/PCL.sup.2)
77/33 21 9
PHB/PCL.sup.2)
49/51 4 18
PHB/PCL.sup.2)
25/75 8 11
PHB/PBA.sup.3)
75/25 32 7
PHB/PBA.sup.3)
49/51 19 4
PHB/PBA.sup.3)
24/76 10 3
PHB/PVAc.sup.4)
74/26 32 2
PHB/PVAc.sup.4)
49/51 29 3
PHB/PVAc.sup.4)
25/75 26 3
______________________________________
.sup.1) PHB: PHB polymer;
.sup.2) PCL: Polycaprolactone;
.sup.3) PBA: Polybutylene adipate;
.sup.4) PVAc: Polyvinyl acetate.
As is apparent from the results shown in Table 4, none of the comparative
blends of a PHB polymer with different polymers other than a racemic PHB
copolymer could significantly improve the stiff and brittle nature of the
PHB polymer.
The principles, preferred embodiments, and mode of operation of the present
invention have been described. The invention, however, is not limited to
the particular forms disclosed, since the details set forth above are to
be regarded as illustrative rather than restrictive. Variations and
modifications may be made by those skilled in the art without departing
from the spirit of the invention.
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